1. Field of the Invention
The present invention relates to a method for separating a sapphire wafer, adapted to manufacture a GaN semiconductor light emitting diode (LED), into chips, and more particularly to a method for separating a sapphire wafer into chips by scribing the sapphire wafer, after grinding and lapping a rear surface of the sapphire wafer and then dry-etching the sapphire wafer, thus allowing the sapphire wafer to be efficiently scribed.
2. Description of the Related Art
Recently, LED displays, serving as visual information transmission media, starting from providing alpha-numerical data have been developed to provide various moving pictures such as CF images, graphics, video images, etc. Further, the LED displays have been developed so that light emitted from the displays is changed from a solid color into colors in a limited range using red and yellowish green LEDs and then into total natural colors using the red and yellowish green LEDs and a newly proposed GaN high-brightness blue LED. However, the yellowish green LED emits a beam having a brightness lower than those of the red and blue LEDs and a wavelength of 565 nm, which is unnecessary for displaying the three primary colors of light. Accordingly, with the yellowish green LED, it is impossible to substantially display the total natural colors. Thereafter, in order to solve the above problems, there has been produced a GaN high-brightness pure green LED, which emits a beam having a wavelength of 525 nm suitable for displaying the total natural colors. The LED display represents a high-quality screen displaying total natural colors having long life span, high brightness and high visibility in accordance with the development of the GaN semiconductor LEDs. Thereby, a large-scale color outdoor LED visual display having a size of 100 inches or more has been proposed, and then developed as an advanced visual information transmission medium, which is connected to a computer to improve a level of outdoor commercial advertisement and displays various visual real-time information including news.
When a blue or green GaN semiconductor LED employed by a color LED display is manufactured, a GaN single crystal is grown on a heterogeneous substrate by a vapor growth method such as an MOCVD (Metal Organic Chemical Vapor Deposition) or HVPE (Hydride Vapor Phase Epitaxy) method, or an MBE (Molecular Beam Epitaxy) method. Here, a sapphire (α-Al2O3) substrate or a SiC substrate is used as the heterogeneous substrate. Particularly, the sapphire substrate is made of a crystal having Hexa-Rhombo (R3c) type symmetry, and has a lattice constant in a direction of a C-axis of 13.001 Å and a lattice distance in a direction of an A-axis of 4.765 Å. Orientation planes of sapphire substrate include a C (0001) plane, an A (1120) plane, an R (1102) plane, etc. Since the C plane of the sapphire substrate has a GaN thin film easily grown thereon, and is low-priced and stable at a low temperature, compared to the SiC substrate, the sapphire substrate is mainly used for the blue or green LED.
Generally, a GaN semiconductor LED comprises a sapphire substrate, a first conductive clad layer, an active layer and a second conductive clad layer. The first conductive clad layer, the active layer and the second conductive clad layer are sequentially formed on the sapphire substrate. The first conductive clad layer includes an n-type GaN layer and an n-type AlGaN layer, and the active layer includes an undoped InGaN layer having a multi-quantum well structure. The second conductive clad layer includes a p-type GaN layer and a p-type AlGaN layer. In order to improve lattice matching between the n-type GaN layer and the sapphire substrate, a buffer layer such as an AlN/GaN layer is formed on the sapphire substrate prior to the growth of the n-type GaN layer on the sapphire substrate. In order to form two electrodes on an upper surface of the sapphire substrate, which is an electrical insulator, the second conductive clad layer and the active layer are etched at a designated area so that an upper surface of the first conductive clad layer is selectively exposed to the outside, and a first electrode is formed on the exposed upper surface of the first conductive clad layer. Since the second conductive clad layer has a comparatively high resistance, an Ohmic contact layer is additionally formed on the upper surface of the second conductive clad layer and a second electrode is formed on an upper surface of the Ohmic contact layer. In an actual manufacturing process, a sapphire wafer is used as the sapphire substrate.
After the first conductive clad layer, the active layer, the second conductive clad later and the electrodes are formed on the sapphire wafer as described above, the sapphire wafer is separated into individual semiconductor chips. Here, since sapphire is a very solid material (having mohs hardness of 9) in physiochemical properties, the rear surface of the sapphire wafer is ground, lapped and polished so that the thickness of the sapphire wafer is reduced, and is then scribed into individual chips using a diamond tip. Thereby, the sapphire wafer is separated into the chips.
FIGS. 1a to 1c are schematic views illustrating a conventional process for separating a sapphire wafer into chips. FIG. 1a shows a grinding step. As shown in FIG. 1a, a rear surface of a sapphire wafer 10 is ground so as to have a designated thickness (for example, 115 μm) using a rotating diamond wheel 11. Then, as shown in FIG. 1b, the ground sapphire wafer 10 is lapped and polished. In a lapping step, the sapphire wafer 10 is mounted on a lapping plate 12 and then ground so as to have a designated thickness (for example, 81 μm) using a diamond slurry 13 having a particle size of 6 μm. After the lapping of the sapphire wafer 10, as shown in FIG. 1b, the sapphire wafer 10 is polished so as to have a designated thickness (for example, 80 μm) using a diamond slurry 13 having a particle size of 3 μm. Then, as shown in FIG. 1c, the polished sapphire wafer 10 is scribed and separated into a plurality of chips using a diamond tip 14.
FIG. 2a is a scanning electron microscopic photograph of the rear surface of the sapphire wafer 10 after the above lapping step, in which considerably deep scratches are formed. Such a rear surface of the sapphire wafer 10 is rough, thus having a reduced degree of clearness. When the above rear surface of the lapped sapphire wafer 10 is scribed, the rear surface of the lapped sapphire wafer 10 has a high processing stress and is easily cut with the diamond tip 14. However, as shown in FIG. 3a, serious cracks are generated on the rear surface of the lapped sapphire wafer 10 due to the deep scratches formed therein, and as shown in FIG. 3b, chips separated from the lapped sapphire wafer 10 are partially broken. Thus, after the lapping step, the rear surface of the sapphire wafer 10 is polished using a diamond slurry having a smaller particle size than that of the diamond slurry used in the lapping step.
FIG. 2b is a scanning electron microscopic photograph of the surface of the sapphire wafer after the above polishing step, in which the scratches shown in FIG. 2a are almost all removed. The scratches formed in the rear surface of the sapphire wafer are almost all removed by the polishing step. Thereby, the sapphire wafer has a high degree of clearness, and a semiconductor light emitting diode made of the sapphire wafer has an improved quality. However, the above rear surface of the polished sapphire wafer is smooth and has a low processing stress. That is, when the above rear surface of the polished sapphire wafer is scribed, the rear surface of the polished sapphire wafer has a low surface resistance and is not easily cut with the diamond tip. FIG. 4a is a photograph of the sapphire wafer scribed into chips with the diamond tip after the polishing step. As shown in FIG. 4a, boundaries of the scribed chips are not clean. When the sapphire wafer is separated into individual chips under this condition, there are generated defects in the shape of the obtained chips as shown in FIG. 4b, thus decreasing yield of the chips. The above-described conventional method increases the quantity of abrasion of the diamond tip, which is an expensive piece of equipment, thus increasing unit cost of chip products.
Accordingly, there is required a method for separating a sapphire wafer into chips, which prevents defects in the shape of the obtained chips and reduces the quantity of abrasion of the expansive diamond tip.